Time delay integration (TDI) is an imaging process that produces a continuous image of a moving two-dimensional object. In a TDI system, image photons are converted to photocharges in an array of pixels. As the object is moved, the photocharges are shifted from pixel to pixel down the sensor, parallel to the axis of movement. By synchronizing the photocharge shift rate with the velocity of the object, the TDI can integrate signal intensity at a fixed position on the moving object to generate the image. The total integration time can be regulated by changing the speed of the image motion and providing more/less pixels in the direction of the movement.
TDI inspection systems can be used for inspecting wafers, masks, and/or reticles. A conventional TDI sensor includes a large array of photo sensor elements (charge-coupled devices (CCDs)) formed as a grid. For example, a conventional TDI sensor could be formed in a 1024×128 array of photo sensor elements. To achieve higher sensitivity than can be provided by using a conventional TDI sensor a plurality of TDI pixels can be arranged in a sub-pixel offset pattern. Sensor interleaving can advantageously increase the resolution and the anti-aliasing capability of a TDI inspection system.
At increasingly smaller technology nodes, it is desirable for the image to be significantly magnified at high resolution, thereby facilitating defect detection. At the same time, faster inspections are being requested, despite the increasing complexity of the wafers/masks/reticles being inspected. To accomplish these goals, the size of the TDI sensor arrays has increased.
Emerging semiconductor fabrication processes demand sensitivity to smaller and smaller particles. Current tools operate on the principle of detecting photons, scattered by defects such as aberrant particles, and differentiating “defect” photons from noise. Noise sources include “noise” photons, scattered by the wafer surface and air, and hardware noise, added to the signal by sensors and electronics. The more photons that are scattered by the defect, and the less noise, the easier it is to detect a defect.
However, the number of photons, scattered by a spherical particle, is proportional to the 6-th power of its diameter. With the same illumination, a 12 nm particle scatters approximately sixty-four times fewer photons than a 24 nm particle. Increasing the number of illumination photons is not an option because of the thermal damage threshold, above which the illumination photons begin to damage the surface.
Existing spot scanning technologies have reached the limit of inspection sensitivity. Technologies in patterned applications have specific implementation details and technology limitations such as available laser power, optical efficiencies, noise sources and XY stage specific implementations that limit inspection speed required for patterned and unpatterned applications.
Consequently, it would be advantageous if an apparatus existed that is suitable for very high resolution, real-time, darkfield wafer and reticle inspection.